1. Technical Field
The present invention relates to an optical unit that receives illumination light and emits projection light reflected from a digital micromirror device, and relates also to a projector provided with such an optical unit.
2. Description of the Related Art
A projector incorporating a conventional optical unit is disclosed in Patent Document 1 identified below. This projector includes a light source, an optical unit, a plurality of digital micromirror devices, and a projection lens. The optical unit includes a total reflection light separation prism and a cross dichroic prism. From the projection side (emergence side) toward the digital micromirror devices, the total reflection light separation prism and the cross dichroic prism are arranged in this order.
The digital micromirror device is a reflective image display element in a rectangular shape as seen in a plan view, and has an image display surface composed of a plurality of minute micromirrors. The digital micromirror device forms an image by producing projection light through intensity modulation on illumination light through ON/OFF control of the inclination of the faces of the micromirrors. Each micromirror pivots about a pivot axis of the digital micromirror device so that it has different angles of inclination in an ON state and in an OFF state.
The total reflection light separation prism is composed of two prisms; one prism has an emergence face, and the other prism has an entrance face and a total reflection face as well as a protruding portion that protrudes toward the emergence face in the emergence direction. Part of the total reflection face is provided in the protruding portion, and white illumination light that has entered the total reflection light separation prism via the entrance face is totally reflected toward the cross dichroic prism. The emergence face is disposed to face the projection lens, and the projection light produce by the digital micromirror devices is emitted toward the projection lens.
The cross dichroic prism has two mutually perpendicular dichroic coat faces; it separates the white illumination light totally reflected from the total reflection face in the total reflection light separation prism into a red, a green, and a blue component, and directs these to the digital micromirror devices respectively. The cross dichroic prism also integrates together red, green, and blue ON-light (projection light) reflected from micromirrors in the ON state in the digital micromirror devices, and emits the color-integrated ON-light toward the total reflection light separation prism. The most emergence face-side face of the cross dichroic prism is parallel to the emergence face, and is perpendicular to the optical axis of the color-integrated ON-light.
In the projector structured as descried above, the white illumination light emitted from the light source and entered the total reflection light separation prism is totally reflected on the total reflection face in it, and then exits from the total reflection light separation prism to enter the cross dichroic prism. The illumination light entered the cross dichroic prism undergoes color separation, so that the red, green, and blue components of the illumination light are emitted to the different digital micromirror devices respectively.
The red, green, and blue ON-light reflected from micromirrors in the ON state in the digital micromirror devices enters the cross dichroic prism to undergo color integration, and is emitted toward the total reflection light separation prism. The color-integrated ON-light passes through the total reflection face in the total reflection light separation prism, and then exits via the emergence face, so that it then passes through the projection lens. Thus, a color image is projected.
Patent Document 1: Japanese Patent Application published as No. 2007-25287 (pages 5 and 6; FIGS. 1 and 3).
In projectors, it is common to move the projection lens in the up-down or left-right direction to change the projection position of the projected image. That is, in projectors, it is common to shift the projection lens up and down or left and rightward. Inconveniently, in the conventional optical unit described above, the total reflection light separation prism has a protruding portion that protrudes in the emergence direction, and the projection lens is disposed to the side of the protruding portion, near it. Thus, the movement of the projection lens in the left-right direction is restricted, and the left-right shift amount of the projection lens (that is, the distance over which it can move left and rightward) is reduced. In a case where the protruding portion of the total reflection light separation prism is disposed under the projection lens, near it, the movement of the projection lens in the up-down direction is restricted, and the up-down shift amount of the projection lens (that is, the distance over which it can move up and down) is reduced. This, disadvantageously, results in diminished usability of the optical unit and the projector.
On the other hand, by moving the protruding portion (total reflection face) of the total reflection light separation prism along the most emergence-side face of the cross dichroic prism in the direction away from the projection lens, it is possible to increase the left-right or up-down shift amount of the projection lens. Inconveniently, in this case, of all the rays of the illumination light traveling toward the total reflection light separation prism, some cross dichroic prism-side rays may not reach the entrance face. Such cross dichroic prism-side rays of the illumination light can be made to reach the entrance face by changing the angle of incidence of the illumination light with respect to the entrance face, but then part of the illumination light is not totally reflected on but transmitted through the total reflection face. This reduces the amount of illumination light that is totally reflected on the total reflection face in the total reflection light separation prism, and reduces the amount of illumination light that strikes the digital micromirror devices, disadvantageously resulting in a drop in the amount of projection light.